1. Field of the Invention
This invention relates to a light beam scanning recording method for recording a continuous tone image on a photosensitive material by scanning a light beam deflected by a rotating polygon mirror on the photosensitive material. This invention particularly relates to a light beam scanning recording method for recording a high-gradation image by eliminating the adverse effects of surface inclination of mirror surfaces of a rotating polygon mirror and variations in reflectivity among the mirror surfaces.
2. Description of the Prior Art
Light beam scanning recording methods wherein a light beam deflected by a light deflector is scanned on a photosensitive material, and the light beam is modulated in accordance with image signals, thereby to record a continuous tone image on the photosensitive material have heretofore been known.
As the light deflector, a rotating polygon mirror is employed in many cases. The rotating polygon mirror is advantageous from the viewpoint of scanning stability over other light deflectors such as a galvanometer mirror.
However, the rotating polygon mirror has the drawback that it exhibits surface inclination of mirror surfaces and deviations in reflectivity among the mirror surfaces, and recording of a high-gradation image is adversely affected thereby. Specifically, in the course of scanning a recording beam by use of the rotating polygon mirror, a single picture element string (a single main scanning line) has heretofore been recorded with the recording beam deflected by a single mirror surface. Therefore, in the case where the mirror surfaces have surface inclination, recording positions fluctuate in the sub-scanning direction among the picture element strings. In the case where the mirror surfaces exhibit variations in reflectivity, the exposure amount differs among the picture element strings even though the recording beam intensity is the same.
Also, in order to stabilize the operation of the rotating polygon mirror, it is necessary to rotate the rotating polygon mirror at a high speed, for example above approximately 3,000 rpm. This requirement also constitutes an obstacle to the recording of an image having a very high level of gradation. Specifically, for example, the recording beam is scanned in the manner mentioned above and is pulse number modulated in accordance with image signals, and the pulse number is controlled within the range of 0 to 1,000 pulses in order to obtain a high level of gradation. In this case, when the number of the mirror surfaces of the rotating polygon mirror is six, the main scanning width is 180 mm, the picture element size is 100 .mu.m (=0.1 mm), and the rotation speed of the rotating polygon mirror is 3,000 rpm, a single main scanning is carried out for EQU 1/(3,000.div.60.times.6)=1/300 second
at the most. Therefore, the frequency of the aforesaid pulses must at least be EQU 180.div.0.l.times.1,000.times.300.times.10.sup.-6 =540 MHz.
Though a semioonductor laser can be turned on and off at such a frequency, the drive circuit for producing the pulses at such a high frequency to control the turning on and off becomes very expensive.